This invention relates generally to techniques used in geophysical well logging, and more particularly to new techniques for automatically processing dipmeter signals to produce more accurate dip and azimuth representations of subsurface formations.
A common method of measuring the dip angle and direction or azimuth of subsurface formations employs a dipmeter tool passed through a borehole drilled into the subsurface formations. This tool may apply any of numerous means to obtain geophysical signals representative of variations of a particular formation characteristic, such as resistivity. One such tool is described in the paper: "The High Resolution Dipmeter Tool", by L. A. Allaud and J. Ringot, published in the May-June 1969 issue of The Log Analyst.
Dip and azimuth measurements representing the inclination of a formation characteristic or feature may be determined from dipmeter signals containing information representing the intersection of such a feature at three or more radially spaced points on the borehole surface. A two-step decision process is traditionally employed whereby in a first step the displacement between two points intersecting a common feature may be determined, under favorable circumstances, by correlating pairs of dipmeter signals, each having a similar response to the common feature. Thereafter, in a second step the displacements between at least three different points are examined to determine the position of a plane. The position of such a plane is conveniently expressed by its dip, an angle measured from a reference (usually horizontal) plane, and its azimuth, an angle measured from a reference direction (usually true North). Typically, the dipmeter signals are recorded as a function of depth on computer compatible magnetic tape at the well site for later processing. The measured signals can be processed either at the well site or off the well site using any of several techniques such as manual, semi-automatic and fully automatic processing which may be aided by either analog or digital computers.
A computer program to perform the digital processing operations is described in a paper entitled "Automatic Computation of Dipmeter Logs Digitally Recorded on Magnetic Tape" by J. H. Moran, et al, and published in the July, 1962 issue of the Journal of Petroleum Technology. An additional computer program is described in the paper, "Computer Methods of Dip Log Correlation" by L. G. Schoonover et al, pages 31-38, published in the February 1973 issue of Society of Petroleum Engineers Journal. Furthermore, programs to process digitally-taped dipmeter data are available from digital computer manufacturers, such as IBM.
Results from the processing of the measured signals are normally presented in tubular listings as dip and azimuth measurements versus borehole depth. When desired, the individual displacements found between the correlation curve pairs which led to the dip and azimuth values may also be presented.
At each step or depth level, one sequence of displacements between various pairs of signal combinations may be obtained. A typical sequence includes at least two displacements but may include a round of up to six displacements in each sequence when four separate signals are employed. When a round or more than two displacements in one sequence is obtained, the displacements may be combined into many more possibly different combinations, each combination corresponding to perhaps a different dip and azimuth measurement. Since only two related displacements are required, it is presently a common practice to utilize only what appears to be the two best qualified displacements. All other displacements are discarded without further consideration, thereby producing only one result per sequence of displacements. Further, little information is retained regarding the position of the sources or of the measured signals of the dipmeter pads corresponding to the utilized displacements other than perhaps a display of a caliper measurement.
When large numbers of measurements result, as from recent high resolution dipmeter techniques, tabular listings are usually augmented by graphic presentations of dip and azimuth representations. The graphic displays vary with the interpretation objective, depending upon whether the purpose is for stratigraphic or structural studies. Accordingly, relationships between the corresponding dip and azimuth measurements and their continuity with depth are considered in different manners.
For stratigraphic analysis purposes, trends of adjacent dip measurements, for example, measurements representing a trend of rapidly increasing dip with depth, are considered separately from measurements representing a trend of rapidly decreasing dip with depth. It is important that the azimuth of these dips remain substantially constant and thereby represent the general direction of sediment transport or perhaps the probable direction of down dip thickening. Dipmeter results may be further combined in a given analysis from intervals corresponding to a given depositional or stratigraphic unit.
Graphic displays used in stratigraphic analysis are typically the azimuth frequency plot (no dip or depth representation) and the Schmidt net and the Stereonet (azimuth versus dip but still no depth representation). These nets and several variations thereof have known statistical characteristics in that they may enhance either low or high dip measurement point groupings. In their use, the dip and azimuth value for each measurement is combined and represented by a point in these nets. A description of some of these displays and their application is given in a paper entitled "Stratigraphic Applications of Dipmeter Data in Mid-Continent" by R. L. Campbell, Jr., published September 1968 in the American Association of Petroleum Geologists Bulletin.
Structural analysis is distinguished from stratigraphic analysis in the type of information needed. While in stratigraphic analysis, the measured signals hopefully represent bedding planes within the boundaries of a given geological unit, these bedding planes have little, if any, regional extent. Structural analysis, in contrast, requires a deliberate attempt to mask out such sedimentary features in favor of enhancing the boundaries of the individual strata.
Conventionally, short lengths (1 to 2 feet) of dipmeter signals along a borehole are correlated to obtain stratigraphic information while long lengths (10 to 20 feet) are correlated to obtain structural information. While use of long correlation lengths to obtain structural dip has been standard practice for some time, there are certain disadvantages associated with the use of long correlation lengths. One such disadvantage is that the use of long correlation lengths masks dip patterns needed for stratigraphic analysis, thus additional computatons must be made using a shorter length to obtain stratigraphic information. Another disadvantage is that most techniques employing long correlation lengths are influenced by frequently occurring stratigraphic features having a common dip and direction, even though each such feature is less pronounced than the structural feature. Thus, the use of long correlation lengths does not assure that accurate structural dip information has been obtained. Yet another disadvantage is that current correlation techniques tend to ignore the possibly objectionable effects of rotation of the dipmeter tool within the long correlation interval. While it would be more desirable to obtain the detailed information availiable only from short correlation intervals and then apply previously mentioned trend analysis to separate the stratigraphic and structural dips, it will be appreciated that as the correlation interval is shortened, the probability of obtaining a completely erroneous displacement increases substantially. The wrong peak on the correlation function produced in the correlation process may be used to determine the displacement. Such invalid displacements may be combined with valid displacements to produce erroneous dip information which add scatter and confuse valid trends or, when the invalid displacements are systematically erroneous, may even appear as false trends. As a compromise, it has been the practice to employ correlation intervals having a length which is greater than the length actually desired so as to reduce this scatter to an acceptable level such that any valid trend which may be present might be found. As a result, the occurrence of dip estimates has no relationship to the occurrence of bed boundaries, deposition boundaries between regions of different geological activity or the degree of geological activity.